1. Field Of The Invention
The invention relates to a hydraulically actuated brake, and more particularly a sliding-caliper disc brake, having a brake shoe and an actuating piston of two-part construction mounted for displacement in a cylinder and acted upon by a pressure fluid.
2. Description Of The Prior Art
In hydraulically actuated brakes, the heat generated by the friction represents a considerable problem because it flows away, inter alia, through the actuating piston bearing against the brake shoe as a result of which, heating of the hydraulic fluid occurs. The temperature of the hydraulic fluid must, however, in no circumstances rise above 120.degree. C. because otherwise vapor bubbles form as a result of which the operational capacity of the brake is influenced considerably.
In order to reduce the flow of heat taking place through the actuating piston, it is already known to construct the actuating piston in the form of a cup-shaped portion of cast steel, the bottom of which faces the pressure-fluid side, and of a cylindrical core of a mechanically resistant and heat-insulating material which is inserted coaxially in and detachably connected to the cup-shaped portion and which can be pressed against the brake shoe. In a hydraulically actuated brake of this type described in German Patent reference DE-OS No. 34 16 942, corresponding to U.S. Pat. No. 4,572,334, the cup-shaped portion consists of cast steel and the core consists of 30 percent by weight of glass fibers, 50 percent by weight of mineral filling material and 20 percent by weight of binder, for example phenolic resin. In this known actuating piston, the core is provided with a central tapped bore and the cup-shaped portion has a corresponding threaded pin onto which the core is screwed. This two-part construction of the actuating piston is still not sufficient, however, for reliable prevention of excessive heating of the hydraulic fluid and hence the formation of vapor bubbles in the event of prolonged brake actuation as for example during a relatively long descent.